Conventional riflescopes and other optical sighting devices often include an objective, commonly a lens or group of lenses, that creates within a sighting device an inverted, or upside-down, image of a real object. An erector system, such as an image-erecting lens group, prism, or set of mirrors, positioned between the ocular and the objective is also often included in such sighting devices. A primary purpose for including an erector system in an optical device is to invert, or flip, the image created by the objective so the real image viewed by a user has the same orientation as the real object.
In some such sighting devices, erector lens elements are movable longitudinally along the optical axis for varying an optical power (magnification) of the sighting device. One such system is described in U.S. Pat. No. 3,058,391 of Leupold. Varying the magnification of the image viewed by the user may make an optical device easier to use, and may provide use of the optical device in a wider range of applications than if the optical power could not be altered. For example, when a user of binoculars, a riflescope, a spotting scope or other suitable optical sighting device scans a distant scene for a particular object, a lower level of optical power provides a wider field of view than a higher magnification device does. In other words, more of the distant scene appears to a user of the optical sighting device at a low magnification. Using a lower magnification setting permits the user to rapidly scan the distant scene to locate the particular object, such as a deer. Once the user locates the particular object and views the object through the optical sighting device, increasing the magnification permits a more detailed view of the object without the need to reacquire the object's position in the distant scene.
Many riflescopes and other optical devices utilizing movable erector lenses for adjusting the optical power of the device may be limited to a zoom ratio of around 4:1 or less, and often close to, or less than 3:1. For example, a variable-power riflescope may have a magnification ranging between 3× and 9× for a zoom ratio of 3:1 (sometimes referred to as a 3× zoom), which means that a distant object appears between three times closer than it actually is to nine times closer than it actually is depending on where the user sets the magnification.
Some riflescopes may have a zoom ratio of around 10:1, but 10:1 appears to be the upper limit zoom ratio for current riflescopes and other optical sighting devices. There are several factors that may limit how high of a zoom ratio current optical sighting devices may have. For example, the range of travel the optical elements may need to traverse for a zoom ratio greater than 10:1 may require a zoom ring that rotates more than 180 degrees. However, designing a zoom ring with more than 180 degrees of rotation may require cutting a large arc from a scope tube that weakens the scope tube.
Another example may be the difficulty of designing a set of cams to move the lenses in an erector for a zoom ratio greater than 10:1. Because the lenses are likely to require a large range of travel and a zoom ring is likely to be limited to 180 degrees, or less, of rotation, the cam profiles may be relatively steep and be prone to binding an erector element instead of moving it. Another likely limiting factor may be the need to incorporate a gear box or other mechanism to provide the needed travel for the erector elements, which would add complexity and cost to an optical sighting device. Another potential problem for current optical sighting devices attempting to include a relatively high zoom ratio, such as 8:1 or greater, may be vignetting, which is a dark ring appearing around the image viewed through the optical sighting device that often reduces the field of view.